1. Field of the Invention
The present invention relates to methods and apparatus for surface and wavefront measurements; more particularly to automated setup, calibration, and acquisition of such measurements; and most particularly, to such methods that improve the accuracy of aspheric surface and wavefront measurements.
2. Discussion of the Related Art
Methods and apparatus for making precision measurements of surfaces and optical wavefronts are well known in the prior art. The preferred devices for obtaining optical-quality measurements are (unsurprisingly) based on optical technology. The most widely accepted metrology instruments in this field are based on the principle of interferometry, though some other technologies are applicable.
Such equipment has demonstrated capability to obtain measurements of exceptional accuracy. A contour map of a part surface of ˜100 mm diameter can be obtained accurately to 10 nanometers or so (between one-tenth and one-hundredth of a wavelength of visible light) by using a Fizeau interferometer or the like. Microscope-based interferometers (e.g. those based on a scanning white-light technology) can accurately measure the height of features to better than a nanometer over ˜1 mm diameter area. The capability of Fizeau and microscope interferometers has been sufficient for all but the most demanding applications. As time goes on, however, application requirements have become more stringent. For example, optical lithography has exacting scattering (“flare”) requirements. These require sub-nanometer accuracy measurements over a range of spatial frequencies that includes the band from 0.1 to 10 mm−1. This band is on the high end of a Fizeau interferometer's lateral resolution capability, but on the low end of an interference microscope.
Commercially available interference microscopes can achieve the height accuracy (or nearly so) of the present application, but lack the lateral range required. The use of a lower-magnification microscope objective increases the lateral range, but this method is effective only for flat (or nearly flat) surfaces. Curved surfaces (either spherical or aspherical) cannot be measured, due to their large deviation from the instrument's reference surface (curved part vs. flat reference).
Commercially available Fizeau interferometers can easily achieve the lateral range. To achieve the necessary resolution (˜100 micrometers), however, the interferometer requires more magnification than is usual. Relatively straightforward optical design modifications can address this issue. A more significant challenge is related to the height accuracy of a Fizeau interferometer, which is usually inferior to that of an interference microscope. The primary factors contributing to the lesser accuracy are the greater coherence of the light, the inability to precisely focus on the test surface, and the greater difficulty calibrating systematic errors. Innovations are needed to enhance the current state of the art. The associated challenges are far more demanding when the surfaces to be measured are aspheric.
As the coherence of the gauge light source increases, the resulting measurements become more sensitive to defects such as scratches, dust particles, speckle, and ghost reflections. Such defects can degrade measurement repeatability and also introduce systematic errors (biases). Various techniques exist in the prior art for effectively reducing the light source coherence to reduce such errors without unduly degrading the performance in other ways. See, for example, U.S. Pat. No. 5,357,341 to Kuechel; U.S. Pat. No. 6,061,133 to Freischlad; and U.S. Pat. No. 6,643,024 to Deck et al. These techniques, however, make the system more sensitive to focusing errors. Furthermore, the resolution of higher spatial frequency surface features is also more sensitive to focusing errors. Thus proper focusing of the measurement system has become very important.
The best focus position depends on the interferometer optics and the radius of curvature of the test part. For a given spherical (or plano) test part, these parameters change only when the interferometer optics are changed. This typically does not occur when testing a part, so the interferometer operator can usually set the focus manually with enough precision to obtain an adequate measurement. Aspheres, however, have two local principal radii of curvature that vary depending on which portion of the aspheric surface is being examined. For any given portion, there is therefore some freedom in designating a nominal radius to test against. The changing nominal radius of curvature means that the optimal test location (with respect to the optics of the interferometer) changes. This change in test object location (conjugate) means that the position of best focus also changes. For aspheres, therefore, the best focus position for the interferometer depends on which portion of the surface is currently being measured.
The prior art method of user-adjusted focus is insufficient to achieve automatic measurements at multiple points on a test asphere (i.e. without the user needing to refocus manually between measurements). An automatic focusing mechanism would also improve measurement reproducibility, since variation in an operator's manual focusing technique would be eliminated. Other devices (such as photographic cameras) employ autofocusing technologies, but these are not directly applicable to the measurement of optical surfaces. So-called ‘passive’ methods rely on optimizing the contrast of structure in the image. Since optical surfaces in general do not have significant surface structure, i.e. they are very smooth, such methods fail (much as they do when trying to focus on any feature-less target, such as a cloudless sky). ‘Active’ autofocusing methods, however, measure the distance to the target with some auxiliary instrumentation, and use knowledge of the optical system to compute the necessary focal position. Although this basic principle is applicable to a wavefront-measuring gauge, there are areas for potential improvement. Since a wavefront measuring gauge already emits illumination (and detects reflected light) as part of its basic function, a means for employing this for the purposes of measuring distance instead of employing an additional system is desirable. Furthermore, the precise optical characteristics of a wavefront-measuring gauge may not be well known. (For example, it may use commercially available lens subassemblies whose precise designs are proprietary.) Thus a method for calibrating the optical parameters of the gauge as they pertain to focus would be a significant improvement.
The calibration of systematic errors with higher spatial frequencies is also an area where improvement is needed. The reference wavefront error (a spatially dependent height bias in the measurement) is a significant accuracy limitation. Several techniques exist in the prior art for calibrating such errors, including two-sphere, random ball, and subaperture stitching with interlocked compensators. See, for example, J. C. Wyant, “Absolute optical testing: better accuracy than the reference”, Photonics Spectra, March 1991, 97-101; C. J. Evans and R. E. Parks, “Absolute testing of spherical optics”, Optical fabrication and Testing Workshop, OSA Technical Digest Series 13, 185-187 (1994); and P. Murphy, J. Fleig, G. Forbes, and P. Dumas, “Novel method for computing reference wave error in optical surface metrology”, SPIE Vol. TD02, 138-140, 2003. None of these methods, however, is suitable for extreme accuracy calibrations of higher-resolution features in the reference wavefront. High spatial frequency wavefront features evolve more quickly when propagating through space than those of low spatial frequency. Thus it is important to calibrate such features at the conjugate position at which the test optic will be measured; otherwise the estimate of the higher spatial frequency content in the reference wavefront will be inaccurate. For example, a ball calibration executed on a sphere with a 50 mm diameter will not accurately calibrate the higher spatial frequencies for a test part with a 200 mm radius of curvature. The radius of curvatures of the calibration and test part are significantly different, and therefore the higher spatial frequency content of the reference wavefront error will also differ.
For spherical test parts, the ball technique can be adapted to accurately calibrate higher spatial frequencies in the reference by simply executing the calibration on a part of approximately the same radius as the test part, or by executing the calibration on the test part itself. The part is likely a section of a sphere (rather than a complete ball), but it will work for this purpose if its size exceeds the measurement area by some margin (approximately the longest spatial wavelength of interest, provided the surface structure is not unduly spatially correlated). If a complete sphere is not used for the calibration, the averaging technique will not necessary converge to the correct value of the reference wavefront, due to spatial correlation in the part's surface profile. This correlation, however, rarely extends to the higher spatial frequencies, allowing them to be calibrated adequately using a part that is not a full sphere. Note that in cases where the lower spatial frequencies are also important, the method that involves subaperture stitching with interlocked compensators will work in tandem with a non-ball average to enable accurate characterization of the reference wave over all measurable spatial frequencies. This stitching technique is not, however, generally applicable to aspheric surfaces. Alternative methods are thus required to obtain calibrations of higher spatial frequency wavefront error on aspheres.
What is needed in the art, therefore, is a method for automatically setting the measurement device focus position, and preferably a method wherein relatively little knowledge of the actual focusing optics is needed.
What is further needed in the art is a method for calibrating or otherwise reducing the systematic errors of the measurement device, particularly those with higher spatial frequencies, when measuring aspheric surfaces.
It is a primary object of the present invention to enable automatic focusing of a metrology system on a test surface when its local radius of curvature is known.
It is a further object of the present invention to improve a wavefront measuring gauge's accuracy, particularly for aspheric surfaces and higher spatial frequencies.